Despite some inconsistencies in study results, evidence of an association between welding fume exposure and occupational asthma has been growing over the past 15 years. Most of the earlier epidemiological studies concerning welding exposures and respiratory effects have been limited by their cross-sectional study design, the selection of study populations with extensive welding experience, and limited information on the timing and nature of exposures to welding fumes. We propose a study of 200 welding apprentices in which airway reactivity and acute respiratory responses will be measured early in the apprentices' careers, within 6 months of their first welding exposure. Since welders who experience respiratory problems due to welding are less likely to stay in the welding field long-term, a study of welding apprentices who are just entering the field has the advantage of including workers who experience respiratory problems early on. The study design focuses on short-term changes in pulmonary function parameters with simultaneous extensive exposure assessment that will include real time measurements of particulates, and (on a subsample) NOx. Average levels of exposure during the training session will be provided for these variables and for fluoride. Recent evidence suggests an acute respiratory response 15 minutes after first daily exposure to welding, so pulmonary function and respiratory symptoms will be measured before the start of a welding training session, 15 minutes after the first weld, and at hourly intervals throughout the welding session. The real time personal exposure measurements will be used to identify peak levels and all measures will contribute to measuring average exposures in the specified time intervals above. These exposure variables will be examined in relation to respiratory responses, and airway reactivity will be measured by response to methacholine before and after apprentices have learned to weld. In addition, determinants of welding exposures will be identified by considering factors such as ventilation, type of welding work, and worker habits. This study will provide insights into the nature of the association between welding exposures and acute respiratory responses. Recommendations will be made for the most effective respiratory and exposure variables to use in epidemiological assessments. Assessment of determinants of exposure will also be useful for consideration of exposure control options.